This invention relates in general to structures such as load bearing structures and more particularly to structures that provide an enhanced trade-off between the stress that can be safely carried in relation to the amount of material required for the structure.
This enhanced strength to weight ratio is the goal of a large number of designs including many of those proposed and constructed by Richard Buckminster Fuller.
In most contexts where load bearing structures are employed and especially in bridges and beams, in arches and trusses, failure occurs because of a failure in tension rather than in compression. Although the loads imposed primarily induce compressive stress in the material, that stress is resolved within the material by vectors which introduce tension. For example, a bridge subject to load on its upper surface will tend to deflect in such a fashion as to introduce a tension along its lower surface. Failure will occur because of a failure in tension.
To compensate for this effect, rods or fibers which are particularly strong in tension can be incorporated. Multiple ply and laminated materials having varied fiber orientation in different layers are frequently employed to resist tension. In some materials and in particular with ceramics, careful attention is paid to minimize grain boundaries where failure tends to appear.
Attention has been paid to developing materials which have great tensile strength for use in load bearing structures in such a way as to employ the tensile strength of these materials so that loads applied to the structure will be resolved, at least in part, by the tension created in these tension members. Such an approach is outlined in the Buckminster Fuller U.S. Pat. No. 3,354,591 issued in 1967. A more recent improvement on that structure is set forth in U.S. Pat. No. 4,207,715 issued in 1980. This combination of tension and compression members is also disclosed in the structure shown in U.S. Pat. No. 4,711,062 issued in 1987.
As a general rule, most cost effective materials of load bearing structures are far stronger in compression than in tension. This is true of concrete, steel and aluminum, for example. The design of structures using such materials is widespread because of availability and reasonable cost.
Accordingly, it is a major object of this invention to provide a structure which has an improved stress resolution performance using a material having greater compression strength than tensile strength.
It is a related purpose of this invention to provide an improved load bearing structure whose critical failure point will be a function of its strength in compression, rather than its strength in tension.
It is a further related purpose of this invention to provide a load bearing structure which for a predetermined failure point will require less material than is currently required by known structures.